The invention relates to a television signal transmitting system comprising a device for precorrecting products of nonlinearity.
The transmission of television signals requires very linear systems, either at the level of the low-noise reception amplifiers, at the level of the intermediate-frequency converters or at the level of the power stages of the transmitter.
In fact, a television signal wich is composed of an amplitude-modulated image carrier and a sound carrier modulated in accordance with the amplitude or frequency standard, must be amplified very linearly for the demodulated signal to be faithful. The nonlinearities obtained in one stage cause impairment of the transmitted signals. There can for example be observed in the image channel a modification of the contrast, of the colors, and in the sound channel a distortion of the low frequencies. They may also cause, in the case where the image and sound signals are amplified simultaneously, extremely troublesome disturbing phenomena due to the modulation transfers or cross-modulation from one carrier to the other which result in giving the images a watery effect, degradation of the signal/noise ratio, etc . . . .
For all these reasons, television transmission and retransmission equipment of low or medium power (1 watt to 1 kilowatt) comprise Class A biased amplifiers, which leads to an efficiency between 5 and 10%. This efficiency may be substantially increased with devices for correcting nonlinearities.
Different types of precorrection are used at the present time, for television transmitters with separate sound and image channels or with common channels, in particular, videofrequency or intermediate-frequency threshold precorrection devices, or devices of the negative feedback type comprising a device reinjecting into the system nonlinearity products obtained by comparison of the signals before and after processing by the output amplifier.
All these solutions have the drawback of being specific to the transmission systems to which they are applied and in particular all these devices are in general fairly difficult to adjust. In fact, several adjustments are required and these adjustments interact.
A particular precorrection device comprising a generator of nonlinearity products has been described in French patent application No. 75 16062 filed on May 23, 1975 in the name of the Applicant. The generator of nonlinearity products has a law of the same nature as the output amplifier. Its energization level is chosen so that it reaches the same distortion point. The input signal U.sub.e is transmitted symmetrically to this generator and to a delay line which equalizes the respective delays of the two channels. The signals supplied by the generator of nonlinearity products of form k'.sub.3 U.sub.e.sup.3, in phase opposition with the nonlinearity products generated by the amplifier to be corrected of form -k.sub.3 U.sub.e.sup.3, are then combined with the signal emerging from the delay line U.sub.e. Thus the amplitude-amplitude curve of the signal, compressed in the output amplifier (because in particular of this nonlinearity term of order 3, -k.sub.3 U.sub.e.sup.3 which is subtracted from the term k.sub.1 U.sub.e of linear amplification), undergoes preliminary expansion by the precorrection device which may counterbalance, after amplification, the nonlinearity products introduced so that the resulting signal has minimum distortion. This type of precorrection presents excellent stability and an efficiency which is naturally variable when the level of the input signal varies. In fact, when the input signal decreases, the distortion of the output stage and at the same time that of the precorrector decrease with the same rapidity. There is then correction in a wide range.
But the amplitude-frequency characteristic of the nonlinearity product generator is not perfectly flat over the whole frequency range, and because of the phase and group delay time dissymmetries of the two channels, a variable phase shifter must be introduced so as to re-establish in a given frequency range a correct symmetry.
Such a precorrector is fairly simple to adjust since it is sufficient to adjust the amplitude of the nonlinearity products generated and the relative phase of the two channels. But these adjustments must be effected by successive approximations with respect to each other and must be carried out after each variation of the intermodulation level.
The precorrection device described above does not then lend itself, from a measurement of nonlinearity products, to a simple automatic correction.